Micromechanical actuators are used in a plurality of applications today. For example, micro-mirrors are utilized in projector units which are to be set up in a very small installation space.
In such projector units, micro-mirrors are usually utilized which represent a so-called MEMS, i.e., micro electro mechanical system. These types of MEMS mirrors often have several mechanical resonance points which may be correspondingly electrically excited and which are also known as modes or poles in the transfer function. Furthermore, such MEMS mirrors also have anti-resonance modes which are also known as zero points in the transfer function or notch.
The modes of the MEMS mirrors are subdivided into useful modes and spurious modes. In particular, the excitation of spurious modes has a negative impact on the quality of the image projected.
The mentioned MEMS mirrors form a so-called inert spring mass system which in a first approximation may be modeled as a second order low pass (PT2 element.) In this configuration, the cutoff frequencies of the inert spring mass system are defined by its first mode.
This type of MEMS mirror may either be operated resonantly on either one or multiple useful modes or may be operated quasi-statically. The quasi-static actuation takes place with the aid of a low-frequency signal and avoids excitation of the modes.
FIG. 9 shows the transfer functions for different MEMS micro-mirrors in a Bode plot. The upper diagram shows the attenuation in dB over the frequency. The lower diagram shows the phase in degrees over the frequency. It is apparent in the upper diagram that the five micro-mirrors, of which the transfer functions are shown, exhibit a plurality of resonance modes and anti-resonance modes. These are shown in the diagram with spikes which spike upward or downward. Furthermore, it is apparent in the lower diagram that the mirrors exhibit different phase responses as a function of the frequency. In particular, several mirrors exhibit a phase response which is, at least up to a certain frequency, between −0° and −180°, while in other mirrors the phase response exceeds beyond −180°.
Usually, two MEMS mirrors are required to construct the image with the aid of MEMS mirrors, one of the MEMS mirrors being actuated resonantly and one of the MEMS mirrors being operated quasi-statically. The MEMS mirror which is being operated resonantly is in charge of the line projection of the images, and the MEMS mirror which is being operated quasi-statically is in charge of the line-by-line image construction. Another possibility is using a 2D mirror which is operated both in a vertical and in a horizontal direction.
The MEMS mirror which is being operated in the quasi-statical mode must be actuated in such a way that the resonance modes of the micro-mirror are not excited.
Usually, an MEMS mirror in quasi-statical mode is actuated using a sawtooth signal as a reference variable to, for example, generate a frame rate of 60 Hz. During this process, the sawtooth signal exhibits, in the frequency range, the multiples of the even and odd harmonics of the base frequency. The diagram in FIG. 10 shows two possible sawtooth signals having different return times as dashed and solid curves. The time is plotted on the x-axis, and the amplitude of the sawtooth signal is plotted on the y-axis. The rising edges in FIG. 10 are those edges which guide the MEMS mirror line by line.
The falling edges represent the return of the MEMS mirror into its initial position. The corresponding sawtooth signal in the frequency range is shown in FIG. 11.
FIG. 11 shows that the sawtooth signal exhibits frequency components at 60 Hz and the multiples of 60 Hz, i.e., 120 Hz, 180 Hz and so on, in the frequency range. When actuating an MEMS mirror using such a type of sawtooth signal, one of the multiples of the base frequency might excite a resonance mode of the respective MEMS mirror.
Usually, the MEMS mirrors in the quasi-static mode are actuated by using linear drivers or digital drivers. The micro-mirrors are controlled in a closed loop to achieve a sufficient accuracy during the actuation or to increase the linear deflection. During this process, different controllers may be used, for example, adaptive PD controllers, current controllers and position controllers in feed forward structure, LMS harmonic controllers, iterative harmonic coefficient determination, and the like. All controllers used have in common that they need a very large system bandwidth and thus require a very high amount of computing power.
Patent document U.S. Pat. No. 7,952,783 B2 discusses a controller working by using the iterative harmonic coefficient determination method.
For example, systems having MEMS mirrors and controllers usually require a controller bandwidth of 1 MHz to control every image line exactly. Furthermore, some of the known controller concepts require additional status information of the MEMS mirror which may actually only be determined with great difficulty or which is very difficult to estimate.
Large system bandwidth and high computing power mean a large space requirement in the IC control integrated circuits, for example, for analog-digital converters, microcontrollers, digital-analog converters, driver stages and the like.